With the deepening of globalization, international trade, travel, expatriating work are becoming more and more common. However, there are different mains voltages (like 110 v, 120 v, 220 v, 230 v and 240 v) in different countries worldwide, which leads to unavailability of electric equipment in different countries or areas. For example, Japanese printing equipment are unavailable in Pakistani factories; when people go on a journey or go on a business trip at a foreign land, they may find that electric equipment taken along are unavailable because they are mismatched to the local mains voltage. Therefore, it is necessary to use a voltage conversion device to match electric equipment to different supply voltages.
The currently conventional voltage conversion technique adopts change of transformer turns ratio for voltage conversion. This technique has certain shortcomings; above all, there is a potential hazard during tap changing of transformers, particularly:                Vin/Vout=Nin/Nout        Vin: input voltage        Vout: output voltage        Nin: number of turns for input voltage        Nout: number of turns for output voltage        
If an input voltage is 110V, the output voltage is required to be 220V, and Nin/Nout shall be ½; under this circumstance, if users use an input voltage of 220v by mistake, the output voltage will be 440V. Obviously, such a high output voltage certainly will burn out electric equipment connected to the voltage output end.
At present, common electric appliances have a rated input voltage range of 93V-127V or 190V-245V. Usually, electric appliances may be subject to overburning if they are unprotected. Voltage in power grid is fluctuant. In some areas, voltage fluctuation is as high as 100V during certain time interval. Electric equipment can easily be damaged by such a high voltage fluctuation. In addition, when an input voltage of 110V is required for conversion of an output voltage of 220V, voltage fluctuation can be magnified twice, which can more easily destroy electric equipment.
Furthermore, when a voltage is lowered, load current in transformer coil is increased. If not equipped with a protector, the transformer running at a low voltage for a long time can be easily burned out or even arouses a fire disaster.
A friendly user interface is very important for product design. Users are relieved to use a product if they know its real-time status and variation trend.
On the basis of abovementioned description, it is necessary to improve the existing automatic voltage conversion system, so as to overcome at least one of shortcomings mentioned above. The invention provides a scheme, aiming at how to conduct automatic voltage conversion, high and low voltage protection, transformer over temperature protection and readable user interface.